Effect of simulated rain, coat length and exposure to natural climatic conditions on the efficacy of a topical formulation of eprinomectin against endoparasites of cattle

A series of five controlled studies involving 114 cattle were conducted in Australia, North America and the United Kingdom to examine the effect of simulated rain, coat length and exposure to natural climatic conditions, on the efficacy of a topical formulation of eprinomectin against nematode parasites of cattle. In all trials infections were induced with a range of bovine nematode species and treatment was applied when the majority of nematodes were mature. In one study, simulated rain was applied to cattle ending one hour before treatment or geginning one, three or six hours after treatment. In a second study cattle had short (1 cm) or long (3–6 cm) haircoats at the time of treatment. Three other studies were conducted usinv cattle housed indoors or exposed to various natural climatic conditions. Nematode counts were determined using standard techniques and the efficacy of treatment was assessed relative to vehicle-treated controls. Regardless of the timing of simulated rain relative to treatment, eprinomectin was at least 99.9% effective ( p < 0.01) against Haemonchus placei, Ostertagia ostertagi, Trichostrongylus axei and Cooperia spp. There were also no differences ( p > 0.10) in efficacy between treatment administered to dry or wet cattle, or treatment administered before or after simulated rainfall. Efficacies against O. ostertagi, T. axei, Cooperia oncophora and Dictyocaulus viviparus were >99.5% ( p < 0.01) regardless of the length of the haircoat at the application site. Exposure of treated cattle to sunshine and precipitation had no effect on anthelmintic efficacy ( p > 0.10) with efficacies of greater than 99.5% being maintained against H. placei, O. ostertagi (adult and fourth-stage larvae), T. axei, Cooperia spp., Nematodirus helvetianus (adult and inhibited fourth-stage larvae) and Oesophagostomum radiatum. These findings indicate that eprinomectin (500 μg/kg) in a topical formulation is a safe and highly effective nematocide for cattle regardless of their coat length and this high level of efficacy is maintained in cattle exposed to a wide variety of climatic conditions.